by John Priday, Chief Technology Officer, Teledyne Labtech
As new RF and microwave systems evolve, there is a greater need for effective thermal management and significantly higher RF performance from PC boards, especially those that serve RF power amplifiers and phased array T/R modules. One of many roles that the PC board must perform is to channel heat from the underside of the semiconductor device through to the chosen heatsinking scheme as efficiently and effectively as possible. The design challenge is how best to accomplish this while achieving the other tradeoffs required, such as RF performance, manufacturability, and cost. This paper reviews thermal management methods and demonstrates the advantages of “coin” technology versus traditional thermal vias.
Designers have traditionally added plated through holes (PTHs) to thermal/ground pads under components to remove heat through the circuit to a thermal sink such as a cold wall. Unless the assembly process includes a step to pre-fill these PTHs with solder, there is an elevated risk that solder will be robbed from under the component into the holes, leading to a potentially unreliable connection.
Another solution often used is to fill these PTHs with a proprietary via plugging paste and plating over the top to provide an uninterrupted ground pad. The plugging pastes typically used are electrically nonconductive and offer relatively low thermal conductivity of about 0.6 W/mK. When compared with the conductivity of copper (400 W/mK) this has little beneficial effect on thermal transfer.
Electrically and thermally conductive paste such as silver-loaded epoxy can be used to fill the thermal PTHs, but the thermal conductivity of these pastes is still very low, about 4 to 30 W/mK. Figure 1a shows an example of a cross-section through a filled and overplated via and Figure 1b shows a typical application with filled thermal vias within the ground pad. There are subtle outlines of the thermal vias in the central large ground pad.
One option for improving thermal conductivity is to increase the plated wall thickness of the PTHs from the standard 25 um to 100 um. More smaller PTHs within a ground pad can provide a more effective thermal path than fewer larger PTHs. However, there are limits to the effectiveness of heat transfer using a traditional ground pad with PTHs.
The results of calculations of four different cases are shown in Figure 2. Starting with a typical case of PTHs with 0.1 mm wall plating thickness, it examines the overall thermal conductance with vias filled with a nonconductive filler (Figure 2 a). Using this as the base case, it then examines increasing the number of vias (Figure 2 b), then changes the filling from nonconductive to conductive silver epoxy of two different thermal conductivities (Figure 2 c and d). As can be seen, the benefit of using silver epoxy instead of standard nonconductive plug paste to fill the vias is limited and generally not worth the additional expense.
For many high-performance applications, the total thermal conductances are not up to the task. A more effective approach is to use copper coins that are integrated into the circuit’s structure. A simple approach is shown in Figure 3a, which is a 6 x 6 mm square coin. A more frequently used approach is to have the coin stepped so that heat is not only efficiently conducted away but also spread. This is shown in Figure 3b, and a cross- sectional photograph of a stepped copper coin is shown in Figure 3c. The larger area of copper provides a larger surface area in contact with the cold wall, providing improved thermal transfer.
Metal-backed circuits offer an excellent solution when large amounts of thermal energy must be dissipated (Figure 4). The metal backing can be copper, aluminum, or brass, as this type of circuit is typically used for solid-state power amplifiers and can be either pre-bonded or post-bonded. In the case of pre-bonded circuits, the substrate is supplied pre-bonded to a thick metal backer. This limits connections tracking to a single layer and presents issues during processing, as machining operations invariably take place after the circuit traces have been formed. Great care must be taken to avoid damaging critical circuit features. The advantage is that this provides an excellent ground plane reference.
The post-bonded alternative is easier to manufacture as the circuit is produced and verified before being attached to a pre-machined and plated metal backer (Figure 5). Post-bonded circuits can have more than a single layer of conductors and the circuit is generally bonded to the metal backer using a conductive adhesive layer. For both pre- and post-bonded circuits, the components that require heat dissipation are mounted directly onto the metal backer through openings within the circuit.
A more complex solution is metal-cored circuits. These can be employed where space is limited, and high isolation between RF and control is required along with thermal management. Heat transferred from components to the core can be through thermal vias or direct contact through cavities within the circuit that the components are mounted on.
Consideration must be given to removing heat from the core, and the circuit substrate is typically machined away from two edges to expose the core so it can be clamped within the chassis to transfer heat. In the case of thermal vias in which holes are blind with diameters of less than 0.2 mm and depths of less than 0.3 mm, the holes can be filled with copper using a blind hole plating process. Figure 6 shows examples.
Another technique developed by Teledyne Labtech for thermal management on small circuits less than 25 cm² where the overall thickness is limited but several devices require thermal management is to employ a machined copper plane with up-stands (pillars). This provides an excellent thermal path and the heat can be distributed efficiently through the thermal plane for transmission to a cold wall (Figure 7).
For very high power applications, metal-backed circuits currently offer the best solution for high-power solid-state RF devices that are flange mounted, although they are not well suited for SMT components requiring thermal management. Where SMT components with high dissipation requirements are used, coins provide an effective solution for thermal management.
If the required power dissipation is less, thick-walled filled vias offer a lower cost alternative to coins. Metal core and machined copper planes are generally only employed where space is limited, and cost is not the overriding factor. There are thermally conductive substrates available for RF applications, but even these generally have rather modest thermal conductivity of typically 1.0 to 1.5 W/mK.